The present invention relates to a fixing apparatus employed in an electrophotographic printer.
This type of fixing apparatus includes a metallic heating roller, and an elastic pressing roller which is pressed against the heating roller. The heating roller contains a halogen lamp or the like and is heated by the radiant heat of this halogen lamp.
A sheet to which a toner image is transferred is made to pass through the region between the heating roller and the pressing roller. At the time, the sheet is heated and pressed, as a result of which the toner image is fixed to the sheet.
In the conventional art, the light radiating from the halogen lamp warms the air inside the heating roller. Since the heating roller is heated in this manner, there is inevitably a loss of energy when light is converted into heat, and the heat cannot be transmitted to the heating roller with high efficiency. Hence, the thermal conversion efficiency is as low as 60-70% and the energy saving characteristic is poor.
Due to the low thermal efficiency, the warm-up operation of the fixing apparatus is inevitably long.
In recent years, therefore, a heater-type fixing apparatus employing a cylindrical heat-resistant film has been put to practical use. The fixing apparatus comprises a heating member including linearly-arranged heating elements, and a heat-resistant film movable in the state where it is in tight contact with the heating member. A sheet to which an image is to be fixed is brought into tight contact with the heating member, with the heat-resistant film interposed therebetween. By moving the heating member together with the heat-resistant film, the thermal energy of the heating member is transmitted to the image through the heat-resistant film.
In the fixing apparatus described above, the linearly-arranged heating elements of the heating member must be controlled in such a manner that the temperature distribution becomes uniform in the longitudinal direction of the heating member. Therefore, apparatuses that have been manufactured must have uniform characteristics, and when operating them, temperature control must be executed with high accuracy. For these reasons, the manufacturing cost is inevitably high. In a high-speed copying machine, the heating member must be a high-power type, and the use of such a heating member is not desirable so as to reduce the power consumption.
In an effort to solve these problems, fixing apparatuses using induction-heating technology have been developed, such as those disclosed in Jpn. Pat. Appln. KOKAI Publications No. 9-258586 and No. 8-76620.
In the apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-258586, a coil assembly is formed by providing a coil around a core extending along the axis of rotation of a fixing roller. The coil assembly generates an eddy current supplied to the fixing roller.
In the apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-76620, a conductive film is heated by a magnetic field-generating means, and a recording medium is brought into tight contact with the heated conducted film, for fixing. A heat-generating belt is sandwiched between a pressing roller and members with which magnetic field-generating means is assembled, in such a manner as to form a fixing nip portion.
In the prior art, however, the fixing roller or the heat-generating belt is heated uniformly without reference to the sizes of sheets. In other words, it is heated uniformly at any width position (the width being perpendicular to the sheet feeding direction). For this reason, when sheets of various sizes are passed through the fixing apparatus, the surface temperature of the fixing roller or heat-generating belt may not be even, and a reliable fixing operation cannot be expected.
The present invention has been conceived in consideration of the above circumstances, and an object of the invention is to provide an induction-heating type fixing apparatus which does not give rise to uneven surface temperature distribution and thus ensures a reliable fixing operation for sheets of various kinds.
A fixing apparatus according to one aspect of the present invention comprises: a fixing device which includes a fixing roller and a pressing roller pressed against the fixing roller, the fixing roller and the pressing roller defining a region through which sheets of various sizes pass while being heated; an inductionheating device which heats the fixing roller by induction heating and which includes first and second excitation coil units provided inside the fixing roller and spaced from each other by a predetermined distance in the axial direction of the fixing roller; and a control device which controls or varies an output applied to the first and second excitation coils in accordance with a surface temperature as measured in the axial direction of the fixing roller, the first and second excitation coil units having widths which are greater than those of sheets of maximal size by 20 to 40 mm.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.